1. Field of the Invention
The present invention generally relates to a vehicle driving force control apparatus for a vehicle in which a pair of drive wheels are driven by a drive torque from an electric motor. Preferably, the electric motor is driven with electric power generated by a generator that is driven by an internal combustion engine. The present invention especially useful in an all wheel drive vehicle in which a pair of main drive wheels are driven by a main drive source, such as internal combustion engine, and a pair of subordinate drive wheels are driven by the electric motor. Thus, the invention is particularly well-suited for a so-called battery-less four-wheel drive vehicle in which the engine drives the generator and the electric power from the generator is supplied to the electric motor.
2. Background Information
In conventional practice, a driving force control apparatus for a vehicle in which the front wheels are driven by an internal combustion engine, the rear wheels can be driven by an electric motor, and a clutch or reduction gear is disposed in the torque transfer path from the electric motor to the rear wheel axle is described, for example, in Japanese Laid-Open Patent Publication No. 11-243608.
In the driving force control apparatus described in Japanese Laid-Open Patent Publication No. 11-243608, the generation of shocks during clutch connection is prevented by connecting the clutch after idling the electric motor such that the rotational speed of the electric motor becomes equal to a speed that corresponds to the rotational speed of the axle shaft when a transition to a four-wheel drive state is made during vehicle travel.